Their mother, Cynthia Marie Randolph, was arrested June 23 after telling investigators she left the children in the car to teach her daughter a lesson. She was charged with two first-degree felony counts of injury to a child causing serious bodily injury.

More charges could be filed as a result of the medical examiner's ruling.

The incident occurred May 26, when deputies were called to Randolph's home just after 4 p.m. west of Lake Weatherford, where temperatures had hit 96 degrees.

Randolph, 24, had reported finding the two children unresponsive in the vehicle. She originally told investigators that she was folding laundry and watching TV for about half an hour before realizing she hadn't heard the kids, who had been playing in an enclosed porch area.

After spending 30-plus minutes looking for them, she said, she found them locked in a vehicle on her property. She claimed they'd entered the car on their own and locked themselves in, and said she broke a car window in an attempt to save them.

In subsequent interviews, Randolph spun "several variations of the events," before implicating herself in a final interview with the Texas Rangers and a Dallas-based senior federal special agent, authorities said.

She told them that she'd found the kids playing inside the car about 12:15 p.m. that day and that her daughter refused orders to exit the vehicle. She then shut the door to teach her daughter a "lesson," thinking "she could get herself and her brother out of the car when ready."

She said she then went back into her home, smoked marijuana and slept for two to three hours. Upon discovering the children, she broke the car window to corroborate her initial story.